1. Field of the Invention
The present invention relates to a turbine blade airfoil for an axial-flow turbine, including an intrados producing a positive pressure between a leading edge and a trailing edge, and an extrados producing a negative pressure, as well as to a turbine blade to which such turbine blade airfoil is applied.
2. Description of the Related Art
A common shape of a trailing edge portion in a conventional turbine blade S for an axial-flow turbine is shown in FIG. 8. More specifically, the trailing edge portion of the turbine blade S shown as being encircled in FIG. 8 includes an arcuate surface St having a trailing edge radius r, an extrados Su extending from an upper end of the arcuate surface St toward a leading edge LE and mainly producing a negative pressure during operation of the turbine, and an intrados S1 extending from a lower end of the arcuate surface St toward the leading edge LE and mainly producing a positive pressure during operation of the turbine. A trailing edge TE of the turbine blade S is defined as a point of intersection between the arcuate surface St and a camber line CL. Therefore, the trailing edge TE of the conventional turbine blade S is not pointed at its end but defined as a point on the arcuate surface St having the trailing edge radius r.
There are conventionally known inventions relating to the shape of a trailing edge portion of a turbine blade, which are described in Japanese Patent Application Laid-open Nos. 57-113906, 7-332007 and 9-125904.
The turbine blade described in Japanese Patent Application Laid-open No. 57-113906 has an arrangement in which the trailing edge portion is curved toward the side of the extrados, or an arrangement in which the curvature of the extrados at the trailing edge portion is larger than that of the intrados. This arrangement ensures that the generation of a shock wave at a transonic speed is controlled to provide an alleviation in load applied to the turbine blade and a reduction in pressure loss.
In the turbine blade described in Japanese Patent Application Laid-open No. 7-332007, a corrugated unevenness is formed at the trailing edge portion. This arrangement ensures that the distribution of flow in a radial direction of the turbine is liable to be interfered, and the proportion of speed loss provided by a wake is decreased, thereby providing an enhancement in performance of flow at each stage of the turbine.
In the turbine blade of the vapor turbine described in Japanese Patent Application Laid-open No. 9-125904, the extrados is notched at a tailing edge portion rectilinearly. This arrangement ensures that a reduction in pressure loss is provided, while ensuring a resistance to erosion due to vibration provided by a vapor flow or due to foreign matters within a vapor flow.
The conventionally known turbine blade S of the axial-flow turbine shown in FIG. 8 exhibits a satisfactory performance in a state in which the flow speed along a blade surface is a high subsonic speed and no shock wave is generated. However, the conventionally known turbine blade S suffers from a problem that if the flow speed at the trailing edge portion reaches a sonic speed, shock waves SW1 and SWu generated from the intrados S1 and the extrados Su at the trailing edge portion cause a reduction in performance. More specifically, the shock wave SW1 generated from the intrados S1 at the trailing edge portion interferes with a boundary layer on the side of the extrados Su of an adjacent turbine blade S to cause the generation of a pressure loss. The shock wave SWu generated from the extrados Su at the trailing edge portion provides a strain or a deformation to a blade cascade of the turbine at a downstream stage to make an enhancement in performance of the entire turbine difficult.
The present invention has been accomplished with the above circumstance in view, and it is an object of the present invention to suppress the shock waves generated from the trailing edge portion of the turbine blade of the axial-flow turbine to the minimum to enhance the performance of the turbine.
To achieve the above object, according to a first feature of the present invention, there is provided a turbine blade airfoil for an axial-flow turbine, comprising an intrados producing a positive pressure between a leading edge and a trailing edge, and an extrados producing a negative pressure, wherein the trailing edge is pointed at its end; the intrados is provided at its rear portion with a flat surface portion connected to the trailing edge; and the extrados has a curved surface portion provided at least at a portion thereof corresponding to the flat surface portion.
With the above arrangement, the trailing edge of the turbine airfoil is pointed at its end; the intrados is provided at its rear portion with the flat surface portion connected to the trailing edge; and the extrados has the curved surface portion provided at least at its portion corresponding to the flat surface portion. Therefore, the flowing of a gas from the intrados toward the extrados at the trailing edge portion can be inhibited to moderate a shock wave generated on the intrados at the trailing edge portion, thereby suppressing the pressure loss to the minimum.
According to a second feature of the present invention, in addition to the arrangement of the first feature, there is provided a turbine blade airfoil for an axial-flow turbine, wherein the angle xcex1 of intersection between the intrados and the extrados at the trailing edge is a right angle or an acute angle.
With the above arrangement, the angle xcex1 of intersection between the intrados and the extrados at the trailing edge is a right angle or an acute angle and therefore, the degree of curvature of the extrados at the trailing edge portion can be decreased to reduce the flow speed, and a shock wave generated on the extrados can be moderated, thereby further decreasing the pressure loss.
According to a third feature of the present invention, there is provided a turbine blade for an axial-flow turbine, which turbine blade is obtained by applying the turbine blade airfoil according to the first or second feature to at least a portion of the turbine blade in a span direction.
With the above arrangement, the turbine blade airfoil according to the present invention and a conventional turbine blade airfoil can be utilized in combination with each other, thereby increasing the degree of freedom in the design of the turbine blade.